Existing apparatus and methods for continuous casting of molten metal use a tundish for dispensing the molten metal on a continuous casting surface. The casting surface usually comprises a cylinder rotating at a constant speed and located closely adjacent the tundish whereby molten metal flows onto the chilled surface where it freezes. As the solidified metal strip passes over the top of the rotating cylinder it begins to contract transversely and longitudinally, thereby it separates from the casting surface and is thrown out radially therefrom. A conventional casting surface usually includes circumferentially extending grooves because of its beneficial heat transfer characteristics; the reasons for the grooved surface are well known and need not be explained.
One problem which exists with existing apparatus is the "dog-bone" effect. That is, the resulting cast strip includes longitudinally extending bumps or ridges at each side edge of the strip. The bumb or increased thickness of the strip is obviously undesirable because the best strip for subsequent processing is one which is completely flat. The humps at the transverse sides of the longitudinally extending strip occur because of the heat transfer characteristics of the rotating cylinder.
When a steady state casting operation is achieved, the cylinder withdraws heat from the molten metal at a constant rate and dissipates heat from all its surfaces in exact proportion to the amount of heat withdrawn from the molten metal. As will be clear, with a steady state condition the hottest part of the cylindrical casting surface is adjacent the periphery, roughly intermediate the sides of the casting strip and approximately at the point on the surface where the change of phase occurs from the molten metal to the solid state. From that point on the casting surface there is a heat gradient in all directions. The parts of the cylindrical surface which are at the lowest temperature are at the ends wich do not contact the molten metal at all. A temperature profile along the cylindrical surface looks something like a conventional bell-shaped curve. The humps on the side edges of the cast metal result from two directional heat dissipation at the cylinder edges and single directional heat dissipation at the center of the cylinder. At the center of the casting surface the heat dissipation only radially. At the edges the heat flows radially and toward the ends of the cylinder. Accordingly, the temperature of the casting surface at its edges will always be lower than the temperature at the center. Because the cylinder near its ends is at a lower constant temperature it freezes the metal more quickly and pulls a larger volume of metal, hence the undesirable side humps. This undesirable characteristic of the cast metal strip is eliminated to a great extent by the apparatus to be described subsequently.
Another problem existing in apparatus currently in use is type 1 and 2 ripples.
Type 1 and 2 ripples are formed in the cast strip as transversely extending humps of increased metal thickness along the cast strip. Particularly in relation to the casting of aluminum, for example 3105 and 3004 alloys, oxides form on the upper surface of the molten metal in the tundish. From time to time parts of this crust of aluminum oxide break off to be carried along on the upper surface of the alloy as it is drawn from the tundish by the rotating cylinder. The broken aluminum oxide crust seems to drag an increased volume of the melt along with it when it is drawn from the tundish and when it freezes it creates a transversely extending ripple in the outer surface of the cast strip. Whether this ripple, referred to as a type 1 ripple, is caused by surface tension of the crust or a temperature differential between the crust and the melt is not exactly clear. In any case, the type 1 ripples do form and the reason is immaterial to this invention. Ways have been devised for minimizing the detrimental affect of type 1 ripples and that is not a part of the invention described herein.
Type 2 ripples appear to be initiated by some oscillating factor which causes the molten metal to be periodically pushed deeper than normal into the circumferentially extending grooves circumscribing the casting surface. The result is a transversely extending ridge on both the bottom of the resulting cast surface and a corresponding larger bump on the upper surface of the cast strip. It is believed that the bumps on the two surfaces are in register because of the resulting increase in heat transfer between the molten metal and the casting surface. Specifically, when the molten metal is pushed down deeper into the circumferential grooves the increased contact area between the molten metal and the casting surface results in greater heat extraction, thereby solidifying a large thickness of molten metal; the upper surface hump is the result.
This problem of type 2 ripples has been a continuing one and no solution was proposed until a very specific observation was made on a particular feed apparatus. That apparatus includes a series of baffles in the tundish to give a more uniform flow of molten metal to the casting surface. The theory of the baffles is that one should baffle the center of the tundish because it naturally flows too rapidly due to the fact that the sidewalls of the tundish will retard edge flow. The surface at the center of a flowing stream always flows fastest because there are fewer obstructions to retard flow. In observing the specific casting apparatus in operation there appeared to be turbulence in the edge areas of the tundish as the molten metal flowed onto the rotating casting surface and an observation of the resulting cast strip showed type 2 ripples in the central portion of the strip but no type 2 ripples at the margins of the strip. Thus the theory was formed that inducing turbulence into the molten metal adjacent to and prior to the time it contacted the rotating casting surface would eliminate type 2 ripples. Accordingly, the structure of the tundish was modified to increase the speed of the flowing metal as it approached the casting surface and this was accomplished by sloping or curving the edge of the tundish adjacent the casting surface to form a lip. This downward slope increases the velocity of the flowing metal with the assistance of gravity. Further turbulence was induced by placing a transverse horizontal bar in the flow path below the surface of the metal closely adjacent the casting surface. This eliminated the type 2 ripples and it was only after additional testing that it was discovered the turbulence was immaterial and ultimately the rod to induce turbulence was removed as other parameters were discovered which could be manipulated to minimize type 2 ripples.